Measurements of electrophysiological signals, such as the electric activity of the heart (ECG), the electrical activity produced by the muscles (EMG), and the electrical activity produced by the brain (EEG) of a human or animal body, are required in medical diagnoses and treatments, in clinical research and more and more in consumer healthcare products as well. Traditionally these signals are measured with electrodes that are connected to the skin via a galvanic contact, often using electrolytic glue. It is a disadvantage of this kind of measurement that it may cause skin irritation during prolonged usage, it may restrict the patient or user from free moving and it typically provides less comfort for the patient or user.
In order to overcome the above disadvantages, contactless measurement of the electrophysiological signals may be used. An example of such a contactless measurement is a capacitive measurement, wherein a capacitor is effectively formed in which the human or animal tissue acts as one of the capacitor plates and an electrode plate of a sensor probe acts as the other capacitor plate. In the capacitive sensing, no galvanic contact to the skin is needed, whereby the need for preparation of the skin of the human or animal body and a sticky patch with conductive gel for establishing a good electrical contact is alleviated. This is in particular advantageous, when a longitudinal measurement has to be conducted.
It is a problem of the capacitive electrophysiological sensors the measurement signal may be influenced significantly by the movements of a user whose electrophysiological signals are to be measured. Such influence of the measurement signal is denoted motion-induced artifact, and it is due to the fact that the coupling capacitance is changing due to skin-electrode distance variation induced by movements of the user, causing deterioration of the measured electrophysiological signal.
The above problem is described in U.S. Pat. No. 6,807,438 and WO2006066566.
U.S. Pat. No. 6,807,438 makes use of the fact that effective capacitance is inversely proportional to the skin-electrode distance and that the variation of capacitance with distance is less sensitive at larger distances. U.S. Pat. No. 6,807,438 discloses reducing motion artifacts by intentionally increasing the gap between the electrode plate and the skin. A disadvantage of this solution is that also the sensitivity of the sensor to the probed electrical potential decreases, i.e. the signal amplitude is sacrificed, due to decreased signal-to-noise ratio, for a reduction of motion artifacts. The motion artifacts may be reduced, but they are not eliminated.
In WO2006066566 a method is described in which an electrical signal of known frequency is injected into the human body. By determining the amplitude variation of the corresponding signal as picked up by the capacitive plate, an estimate can be made of the variation of the distance of the two capacitor plates. In this way a variation in the skin-electrode distance can be detected and possibly its impact on measured signals can be compensated for. A disadvantage of this method is that safety issues might play a role related to the injection of currents into the body.
Another investigated approach for motion artifact reduction is the use of motion-sensing technologies, e.g. accelerometers, to measure the amount of motion at the ECG electrode site and to use adaptive filtering techniques to reduce or even remove the motion artifact from the electrophysiological signal. A disadvantage of this approach is that the transfer function from measured quantity, viz. the acceleration, towards electrode-skin distance can be allowed to vary only slowly with time compared to both the electrophysiological signal and the electrode-skin distance itself. This transfer function is dependent on the mechanical configuration of the sensor to skin attachment. The transfer function of an elastic attachment may be constant over time, if no other parts can mechanically interfere. However, if for example clothing is worn over the sensor, it may exert pressure on the sensor, which changes the transfer function accordingly in an unpredictable way. These transfer function changes are induced by movements of the person and therefore typically occur at similar time scales as the changes of electrode-skin distance. Hence the adaptation time constant in an adaptive filtering approach would ideally have to be in the same order as that of the movement, making it prone to instability and/or lack of precision.
Hence, an improved system for compensation for motion artifacts in capacitive measurement of electrophysiological signals would be advantageous, and in particular a more efficient and/or reliable system would be advantageous.